1. Field of the Invention
The present invention relates to a method and system for Fourier transforming 3 dimensional MR data in predetermined position of interest by using phase-correction in MRI (Magnetic resonance imaging).
2. Description of the Related Art
In conventional MRI, a data acquisition block 20 shown in FIG. 1 detects an MR signal generated in a 3 dimensional region of interest using a pulse sequence shown in FIG. 2 to obtain 3 dimensional MR data P1. The 3 dimensional MR data P1 is subjected to 3 dimensional FFT processing by an FFT (Fast Fourier transformation) processing block 21, thus acquiring a 3 dimensional MR image P2.
Gx, Gy, and Gz in FIG. 2 represent gradient fields in a read direction (x-axis direction), an encoding direction (y-axis direction), and a slice direction (z-axis direction), respectively, as shown in FIG. 3. Note that the field Gz has a slice field Gsl and a phase-encoding field Gph. A 3 dimensional data matrix is represented by Nx.times.Ny.times.Nz, and Nz will be simply represented by N hereinafter.
If "resolution" in the z-axis direction is assumed by .DELTA.z and "band" is assumed by Z, .DELTA.z and Z are represented by the following expression using .DELTA.G, T, and G in the pulse sequence shown in FIG. 2. EQU .DELTA.z=1/(N.multidot..DELTA.G.multidot.T)=1/(2.multidot.G.multidot.T) (1) EQU Z=1/(.DELTA.G.multidot.T) (2)
Therefore, when this pulse sequence is used, aliasing occurs in the band Z as a period due to the characteristic of DFT (Discrete Fourier transformation).
As shown in FIG. 3, when a band Z2 having a center position O in the z-axis direction as a center is set as a region of interest, MR signals are also generated in adjacent bands Z2 and Z3. Therefore, an MR image in which adjacent MR images are partially superposed on a subject MR image is acquired. This is called "aliasing". When only the band Z1 is selectively excited, superposition between the adjacent MR images and the subject MR image can be prevented. However, when a portion of interest is present at a position shift from the center position of imaging, i.e., the center of a gradient field and 3 dimensional FFT processing is performed after the portion of interest is selectively excited, an MR image may be irregularly reconstructed or a slice slightly shifted from a predetermined slice may be reconstructed.
Thus, a demand has arisen for an apparatus which can obtain a 3 dimensional MR image on a predetermined slice without being influenced by aliasing in 3 dimensional Fourier transformation processing.